This invention relates to nickel-iron base alloys, and, more particularly, to an alloy containing nickel, iron, chromium, molybdenum, titanium, aluminum and columbium critically balanced to provide good sulfidation resistance combined with a high degree of hot strength at elevated temperatures in the heat treated condition.
A number of alloys have hitherto been developed which were suitable for use under conditions requiring good hot strength and corrosion resistance at the elevated temperatures encountered in internal combustion engines. With the increasing use of fuels containing larger amounts of sulfur, it is becoming more important that such alloys also have good resistance to sulfidation. Thus, at the present time, heavy duty diesel engines, which may burn high sulfur content fuels, require valves and valve components made of an alloy which not only has good hot strength at operating temperatures of up to about 1500.degree.F, but also has high resistance to sulfidation at such elevated temperatures. Alloy A, having a nominal composition of about 15% chromium, 7% iron, 2.5% titanium, 1% aluminum, 1% columbium and the balance nickel, has been used as a valve alloy for diesel engines because of its high strength in the 1300.degree.-1500.degree.F temperature range. However, as the sulfur content of fuel has increased, Alloy A has shown poor resistance to sulfidation attack. This is a type of corrosion in which sulfides form at the surface of the alloy part, and, especially when chromium is removed from the alloy matrix by this sulfidation corrosion, can result in the catastrophic failure of the part.
Alloy B, having desirable properties for use under stress in a sulfur-bearing atmosphere at elevated temperatures has a nominal composition of about 0.05% carbon, 0.30% manganese, 0.20% silicon, 29% chromium, 46% nickel, 20% cobalt, 2.30% titanium, 1.20% aluminum, 0.70% columbium, 0.006% boron and 0.50% maximum iron. The good hot strength at elevated temperatures and high resistance to sulfidation exhibited by this alloy make it especially desirable for use in making valves for diesel engines. However, the high percentage of cobalt and the relatively high expense involved in using iron-free alloying additions make this alloy relatively expensive.
Alloy C has a nominal composition of about 27% chromium, 37% nickel, 8% manganese, 2% titanium, 1% aluminum and 25% iron. This alloy is more resistant than Alloy A to sulfidation attack, but has a much lower strength in the 1300.degree.-1500.degree.F temperature range. For this reason, Alloy C is not a good material for parts which must operate at such temperatures in diesel engines.